In the beginning, there was the Google (SOAP) Search API. In December 2006, Google no longer issued any new keys for this API. Those with keys already could still use the API, but it was effectively deprecated.
Google Photos is one of Google's best efforts. The cloud-based service not only backs up photos, but taps into Google Assistant for help creating collages, albums, and more. The new Google Photos Library API will allow developers to port Google Photos, with all its tools, into their own application.